Apparatus and method for forming an oxynitride insulating layer on a semiconductor wafer

ABSTRACT

An apparatus and method of forming an oxynitride insulating layer on a substrate performed by putting the substrate at a first temperature within the main chamber of a furnace, exposing the substrate to a nitrogen containing gas at a second temperature which is higher than the first temperature, and growing the oxynitride layer on the substrate within the main chamber in the presence of post-combusted gases. The higher temperature nitrogen containing gases are combusted in a chamber outside the main chamber. The higher temperature is in the range of 800 to 1200° C., and preferably 950° C. In a second embodiment, distributed N 2 O gas injectors within the main chamber deliver the nitrogen containing gas. The nitrogen containing gas is pre-heated outside the chamber. The nitrogen containing gas is then delivered to a gas manifold that splits the gas flow and directs the gas to a number of gas injectors, preferably two to four injectors within the main process tube. Gas injection orifices on the order of several millimeters then distribute the pre-decomposed gas to the wafers, producing a more uniformly N-doped wafer load in a batch furnace.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the fabrication ofsemiconductor wafers. More particularly, the invention relates to anapparatus and method for employing nitrogen oxide (NO) or N₂O gases todope silicon dioxide in a furnace at high temperatures to form oxidesand oxynitrides layers on a semiconductor for gate dielectrics.

[0003] 2. Description of Related Art

[0004] The ability to use silicon dioxide (SiO₂) as the gate dielectricmaterial becomes extremely difficult for thickness (t_(ox)) less thantwenty (20) angstroms (Å). Thickness as small as 20 angstroms arerequired for device scaling with channel lengths less than 0.25 microns(mm). For thickness less than twenty angstroms, leakage currents mayapproach 1 A/cm². This is significant when compared to leakages on theorder of 1(10⁻¹²) A/cm² for thickness t_(ox) greater than 40 angstroms.Thus having a thickness on the order of twenty angstroms may produceprohibitive power consumption of the transistors in the off-state, andreliability concerns through lifetime degradation, i.e., devicelifetimes less than ten (10) years.

[0005] The leakage current is caused by direct tunneling of electronsfrom the polysilicon gate electrode through the gate oxide. Boron fromthe doped poly-silicon gate easily penetrates the thin SiO₂ layercausing large V_(t) shifts and more reliability problems. Boron dopedpoly-silicon gate electrodes are required to avoid depletion effectswhich will also cause large V_(t) shifts and higher threshold voltages.Silicon oxynitrides (SiOxNy) or nitrogen (N) doped SiO₂ have been chosenby most integrated circuit chip manufacturers as the material of choiceto replace SiO₂ for gate dielectrics in the thickness range of 15 to 20angstroms. The beneficial effects of nitrogen incorporation aredependent upon the magnitude of the doping and the distribution of thedoping profile relative to both the Si/SiO₂ interface and thepoly-silicon gate/SiO₂ interface. If the nitrogen doping is engineeredcorrectly, leakage current and boron penetration can be reduced, whileminimizing or negating the impact on threshold voltage V_(t) and channelelectron mobility. Additionally, hot-electron defect generation in thesilicon channel can be reduced by nitrogen gettering of hydrogen (H).These effects make scaling the gate dielectric down to fifteen (15)angstroms viable, while minimizing the impact on process integrationthat would occur by changing the gate dielectric to a high dielectricconstant (K) material system; the high-K material being differentmaterial than the SiO₂. The ability to correctly engineer the nitrogendopant profile is absent in the prior art.

[0006] Oxides and oxynitrides for gate dielectrics are typically grownin atmospheric (or reduced pressure) furnaces, where the gas ispre-combusted through a torch injector pre-tube outside of the mainprocess tube. The resultant product is then delivered to the mainprocess tube for reaction with wafers that are pre-processed up to thegate dielectric layer. Typical torch combustion chambers are engineeredto preheat gas up to 850° C. for combustion of O₂ and H₂ to form H₂O andO₂. H₂O is a critical reactant for wet oxidation in the formation ofhigh quality gate oxides, and has been used extensively as such by thesemiconductor industry. The torch is also used to combust chlorinecontaining sources to provide high purity atomic chlorine that is usedas a metal getterer in the furnace process chamber which, along withnitrogen oxide (NO), are the two gases that can be used to thermallygrow or anneal high quality oxynitride films. However, torches have notbeen engineered for N₂O combustion. For example, in U.S. Pat. No.6,017,791, issued to Wang, et al., entitled, “MULTI-LAYER SILICONNITRIDE DEPOSITION METHOD FOR FORMING LOW OXIDATION TEMPERATURETHERMALLY OXIDIZED SILICON NITRIDE/SILICON OXIDE (NO) LAYER,” a methodfor forming a silicon nitride/silicon oxide (NO) layer within amicroelectronics fabrication was introduced. In order to form theselayers, Wang uses two deposition methods within the same depositionreactor chamber, with the first deposition method separated from thesecond deposition method by a vacuum purge of the deposition reactorchamber to assure that the second silicon nitride layer is formed as adiscrete silicon nitride layer upon the first silicon nitride layer.Each of the nitride layers in the Wang art are formed through a chemicalvapor deposition (CVD) process. By implementing a higher temperatureoutside the main process tube (chamber), the instant inventioneliminates, among other things, the vacuum purge of the depositionreactor chamber.

[0007] Nitrogen oxide is not typically used since it requires additionalsafety apparatus due to its toxicity. It can also produce certainundesirable electrical properties of the device, such as low electronchannel mobility, large voltage threshold shifts, and hot-electrondegradation effects.

[0008] The present invention introduces a thermal nitrogen dopant tunerand its ability to make use of the N₂O decomposition mechanisms tofabricate transistors with thickness in the ten to twenty angstromregime.

[0009] Bearing in mind the problems and deficiencies of the prior art,it is therefore an object of the present invention to provide anapparatus and method for utilizing N₂O decomposition mechanisms to maketransistors with thickness as small as ten to twenty angstroms.

[0010] It is another object of the present invention to provide anapparatus and method for using SiO₂ as a gate dielectric material forthickness on the order of 10-20 Å.

[0011] A further object of the invention is to provide an apparatus andmethod for having a substrate with a gate dielectric thickness less than20 Å without producing prohibitive power consumption due to leakagecurrent losses.

[0012] It is yet another object of the present invention to provide anapparatus and method for developing nitrogen doped SiO₂ substrate layersto replace SiO₂ layers as gate dielectrics with thickness on the orderof 10-20 Å.

[0013] Another object of the invention is to provide an apparatus andmethod for reducing leakage current and boron penetration of the SiO₂layer while negating the impact on threshold voltage and channelelectron mobility.

[0014] A further object of the invention is to provide an apparatus andmethod for scaling gate dielectrics down to the 10-20 Å regime whileminimizing the impact on process integration.

[0015] Still other advantages of the invention will in part be obviousand will in part be apparent from the specification.

SUMMARY OF THE INVENTION

[0016] The above and other advantages, which will be apparent to one ofskill in the art, are achieved in the present invention which isdirected to, in a first aspect, a method of forming an insulating layeron a substrate comprising pre-combustion of nitrogen containing gasoutside a furnace having a main chamber, the pre-combustion performed ata temperature higher than that within the main chamber, wherein thenitrogen containing gas includes N₂O or NO.

[0017] In a second aspect, the instant invention is directed to a methodof tuning the magnitude of nitrogen doped profiles for an oxynitrideinsulator on a substrate, the method comprising: providing the substrateat a first temperature within a main chamber; exposing the substrate toa nitrogen containing gas at a second temperature which is higher thanthe first temperature; and, growing the insulator on the substrate inthe presence of the gas. The exposing step further comprises heating thenitrogen containing gas to the second temperature in a second chamberoutside the main chamber and directing the heated nitrogen containinggas to the main chamber. The method further comprises heating chlorineand steam gas to a combustion temperature and transferring the chlorineand steam gas to the second chamber. The heating is accomplished byapplying a torch heating element to the chlorine and steam gas in athird chamber separate from the main and second chambers. The secondtemperature is in the range of 800 to 1200° C., preferably 950° C. Thefirst temperature is in the range 600 to 1100° C., or preferably in therange 600 to 800° C. The second temperature is sufficient to react withthe gas before it reacts with the substrate. The second temperature isadjusted to tailor an amount of nitrogen concentration in the insulatinglayer.

[0018] In a third aspect, the instant invention is directed to a methodfor employing a nitrogen containing gas to form an insulation film on asemiconductor, comprising: combusting chlorine and steam gas in a firstchamber; transporting the combusted chlorine and steam gas to a secondchamber; applying heat to the second chamber to the chlorine and steamgas; removing the chlorine and steam gas from the second chamber to athird chamber, and introducing nitrogen containing gas to the secondchamber from the first chamber; applying heat to the second chamber tothe nitrogen containing gas; transferring the nitrogen containing gas toa third chamber containing the semiconductor; and, applying heat to thethird chamber at a temperature level sufficient to initiate agas-semiconductor reaction wherein a the insulation film is formed onthe semiconductor.

[0019] In a fourth aspect, the instant invention is directed to a methodfor distributing a nitrogen containing gas to form an insulation film ona semiconductor, comprising: providing the semiconductor within a mainprocess tube; pre-heating N₂O gas outside the main process tube;delivering the N₂O gas to a gas manifold; splitting the N₂O gas flowthrough the gas manifold to direct the N₂O gas to gas injectors attachedto the main process tube; and, distributing the N₂O gas through theinjectors to the main process tube housing the semiconductor.

[0020] In a fifth aspect, the instant invention is directed to anapparatus for forming an insulating layer on a substrate comprising: afurnace including: a first chamber having a first heating element forcombusting chlorine and steam gas at a first temperature; a secondchamber having a second heating element, the second chamber adapted toreceive the chlorine and steam gas and separately a nitrogen containinggas at a second temperature; and, a third chamber having a third heatingelement at a third temperature adapted to react separately the gasesfrom the second chamber with the substrate to grow the insulating layer.The second temperature is higher than the third temperature, such thatthe combined gases from the second chamber enter the third chamber at ahigher temperature than the third temperature. The first heating elementcomprises a torch. The second heating element may also be comprised of atorch.

[0021] In a sixth aspect, the instant invention is directed to anapparatus for distributing a nitrogen containing gas to form aninsulation film on a semiconductor, comprising: a furnace main processtube adapted for gas injection within the tube and securing thesemiconductor; a heating element outside the main process tube forpreheating a nitrogen containing gas; a gas manifold adapted to receiveand deliver the pre-heated nitrogen containing gas to gas injectorsattached to the main process tube; the gas injectors adapted todistribute the nitrogen containing gas to the semiconductor. The gasinjectors include at least two injectors within the main process tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The features of the invention believed to be novel and theelements characteristic of the invention are set forth withparticularity in the appended claims. The figures are for illustrationpurposes only and are not drawn to scale. The invention itself, however,both as to organization and method of operation, may best be understoodby reference to the detailed description which follows taken inconjunction with the accompanying drawings in which:

[0023]FIG. 1 is a graph of a comparison of capacitance-voltage curvesfor a thermal nitrogen dopant tuner process and a process using SiO₂control.

[0024]FIG. 2 represents I-V curves for the thermal nitrogen dopant tunerfilm and the SiO₂ control.

[0025]FIG. 3 is a block diagram representing the gas flow through theseparate chambers used in the gas flow/reaction sequence of the instantinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(s)

[0026] In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-3 of the drawings in which likenumerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

[0027] A torch or pre-combustion furnace chamber that is engineered forhigh temperature combustion is used to tune the magnitude and positionof the nitrogen doping profile for oxynitrides. Tuning of the nitrogenprofile in an oxynitride gate dielectric can lower the leakage currentand boron penetration in a CMOS or BiCMOS structure, while maintainingthe same electron channel mobility and keeping the threshold voltageshift to an allowable level. The process of using a torch orpre-combustion furnace chamber for N₂O combustion has been designated“Thermal Nitrogen Dopant Tuning” (TNDT) and the torch or pre-combustionfurnace chamber referred to as the “thermal nitrogen dopant tuner.”

[0028] The thermal nitrogen dopant tuner consists of a high temperatureheating element that can operate in the temperature range ofapproximately 800° C. to 1200° C. This heating element is used for thepre-combustion of N₂O outside the main chamber or wafer chamber. Thelocation of the heating element prevents combustion of the N₂O in thewafer process chamber. If combustion of the N₂O is performed in thewafer process chamber, thickness non-uniformities of the oxynitride filmwill result, due to the exothermic nature of the N₂O gas phasedecomposition. An exothermic reaction will cause NO, O₂, and O topropagate towards the exhaust outlet and across the wafer surface (fromthe outer wafer radius to the inner wafer radius) as reactivecomposition components, both within the wafer and within the loadnon-uniformities. The temperature range of the heating element iscritical to the concentration of NO, which is the reactant thatincorporates N in the grown film, and increases as the temperatureincreases. For temperatures greater that 950° C., enough NO₂ bi-productcan be produced to react with atomic O, thus decreasing the amount ofatomic O compared to increasing the percent of NO composition in the gasphase. This decreases the amount of nitrogen (N) removal at theSi/SiOxNy oxynitridation front, causing the doping concentration at thisinterface to be increased compared to the concentration at lowerpre-combustion temperatures. This also allows for independent control ofthe wafer temperature and gas temperature while providing a plentifulsource of NO from N₂O to form oxynitride. The effect is especiallyadvantageous for device structures which require a low thermal budget.

[0029] An example film and device structure have been demonstrated usinga thermal nitrogen dopant tuner with the desired temperature of 950° C.and a main process tube (main chamber) temperature of 800° C. Electricalthickness uniformities within-wafer (measured using acapacitance-voltage technique) were 0.02 to 0.06 Å. FIG. 1 shows acomparison of capacitance-voltage curves for a thermal nitrogen dopanttuner process 10 and a process using SiO₂ control 12 (without a thermalnitrogen dopant tuner). In generating FIG. 1, sixteen devices weremeasured on both wafers. As indicated by the FIG. 1 curves, excellentuniformity on the order of 0.3 Å or approximately one statisticaldeviation, and small flatband shift on the order of 25 meV, areexhibited by the thermal nitride dopant tuner oxide. Across loadthickness, uniformities were approximately within 0.12 Å for a waferload of one hundred. Nitrogen incorporation in the film was measured atan integrated concentration of 4(10¹⁴) to 5(10¹⁴) atoms/cm². Oxynitridesformed with N₂O under (typical) pre-combustion temperatures of 800° C.to 850° C. yielded nitrogen concentrations of 1(10¹⁴) to 2(10¹⁴)atoms/cm². Oxynitrides formed with NO yielded nitrogen concentrations of6(10¹⁴) to 8(10¹⁴) atoms/cm².

[0030] This increase in N incorporation using the thermal nitrogendopant tuner resulted in a two-fold improvement in leakage current,0.005 A/cm² (at 1 volt reverse bias), versus a leakage current of 0.01A/cm² without the implementation of the N₂O thermal nitrogen dopanttuner. FIG. 2 represents I-V curves for the thermal nitrogen dopanttuner film 14 and the SiO₂ control 16. As indicated by these curves, theleakage current that resulted from the implementation of the thermalnitrogen dopant tuner is reduced from that produced by the SiO₂ controlprocess. This leakage current improvement by a factor of two to threetimes corresponds to the gain of the oxide thickness (or capacitance) byapproximately 0.5 to 1 Å. This leakage current is comparable to thatproduced by oxynitride growth using NO as the gas source. However, foran NO gas source, the mobility and transconductance are degraded due totoo much N incorporation. Implementing the N₂O thermal nitrogen dopanttuner exhibited negligible mobility degradation. In addition, theflatband voltage shift was on the order of 20 meV, which remains anacceptably small value.

[0031] The leakage current problem has been identified by others skilledin this art, and solutions have been proposed using thermal growth oranneal to form oxynitride, or N₂ ion implant into the silicon channelwith a subsequent reoxidation to form the nitrogen-rich interfaciallayer. The implementation of the thermal nitrogen dopant tuner of thepresent invention would serve as a replacement of the N₂ ion implantprocess step(s), or in the alternative, enhance its effect on leakagecurrents. Having the thermal nitrogen dopant tuner external to the waferprocess chamber, thickness and dopant non-uniformiity defects that occurduring decomposition of N₂O in the wafer chamber are no longer produced.The introduction of this tuner in the process also mitigates thetoxicity problem that results from using NO, by creating a gas streamthat has less NO by-products than pure NO.

[0032] The method of forming an insulating layer on a substrate usingthe thermal nitrogen dopant tuner requires pre-combustion of the N₂Ooutside the main chamber at a temperature higher than that within themain chamber. Preferably, the main chamber is kept at a temperature inthe range of 800° C., while the pre-combustion chamber is in the rangeof 950° C. FIG. 3 is a block diagram representing the gas flow throughthe separate chambers used in the gas flow/reaction sequence of theinstant invention. Uncombusted gas is applied to a first chamber 20where a torch is used to combust chlorine and steam. This gas 22 is thentransported to a pre-combustion chamber 24 for N₂O combustion. Thecombusted gas 26 is then delivered to a main chamber 28 for thegas/wafer reaction where an oxynitride film is formed on the wafer. Thesubstrate remains at a temperature within the main chamber where it isexposed to the nitrogen containing gas from the pre-combustion chamber,delivered at a second temperature which is higher than the temperaturein the main chamber. The resultant insulator is then grown on the waferin the presence of this gas.

[0033] A second embodiment of the invention considers using distributedN₂O gas injectors within the main chamber or process tube. The N₂O gasis pre-heated outside the chamber, as similarly required in the firstembodiment. Next, the N₂O gas is delivered to a gas manifold that splitsthe gas flow and directs the gas to a number of gas injectors,preferably two to four injectors, within the main process tube. Gasinjection orifices on the order of several millimeters then distributethe pre-decomposed gas to the wafers, producing a more uniformly N-dopedwafer load in a batch furnace. This apparatus geometry and method ofimplementation is unique to the application of N-doped SiO₂ oroxynitride.

[0034] While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

[0035] Thus, having described the invention, what is claimed is:

1. A method of forming an insulating layer on a substrate comprisingpre-combustion of nitrogen containing gas outside a furnace having amain chamber, said pre-combustion performed at a temperature higher thanthat within said main chamber.
 2. The method of claim 1 wherein saidnitrogen containing gas includes N₂O.
 3. The method of claim 1 whereinsaid nitrogen containing gas includes NO.
 4. A method of tuning themagnitude of nitrogen doped profiles for an oxynitride insulator on asubstrate, said method comprising: providing said substrate at a firsttemperature within a main chamber; exposing said substrate to a nitrogencontaining gas at a second temperature which is higher than said firsttemperature; and, growing said insulator on said substrate in thepresence of said gas.
 5. The method of claim 4 wherein said exposingstep further comprises heating said nitrogen containing gas to saidsecond temperature in a second chamber outside said main chamber anddirecting said heated nitrogen containing gas to said main chamber. 6.The method of claim 5 further comprising heating chlorine and steam gasto a combustion temperature and transferring said chlorine and steam gasto said second chamber.
 7. The method of claim 6 wherein said heatingchlorine and steam gas comprises applying a torch heating element tosaid chlorine and steam gas in a third chamber separate from said mainand second chambers.
 8. The method of claim 5 wherein said secondtemperature is 950° C.
 9. The method of claim 5 wherein said firsttemperature is in the range 600 to 1100° C.
 10. The method of claim 5wherein said first temperature is in the range 600 to 800° C.
 11. Themethod of claim 4 wherein said nitrogen containing gas comprises NO orN₂O.
 12. The method of claim 4 wherein said second temperature issufficient to react with said gas before it reacts with said substrate.13. The method of claim 4 wherein said second temperature is in therange of 800 to 1200° C.
 14. The method of claim 4 further comprisingadjusting said second temperature to tailor an amount of nitrogenconcentration in said insulating layer.
 15. The method of claim 14wherein said second temperature is in the range of 800 to 1200° C. whilethe first temperature is in the range of 600 to 1100° C.
 16. A methodfor employing a nitrogen containing gas to form an insulation film on asemiconductor, comprising: combusting chlorine and steam gas in a firstchamber; transporting said combusted chlorine and steam gas to a secondchamber; applying heat to said second chamber to said chlorine and steamgas; removing said chlorine and steam gas from said second chamber to athird chamber and introducing nitrogen containing gas to said secondchamber from said first chamber; applying heat to said second chamber tosaid nitrogen containing gas; transferring said nitrogen containing gasto a third chamber containing said semiconductor; and, applying heat tosaid third chamber at a temperature level sufficient to initiate agas-semiconductor reaction wherein a said insulation film is formed onsaid semiconductor.
 17. The method of claim 16 wherein said nitrogencontaining gas includes nitrogen oxide (NO) or N₂O.
 18. The method ofclaim 16 wherein combusting said chlorine and steam gas in a firstchamber comprises applying heat to said chlorine and steam gas from atorch heating element.
 19. The method of claim 16 wherein forming saidinsulation film on said semiconductor includes growing said insulationfilm in the presence of said combusted chlorine, steam, and nitrogencontaining gases.
 20. The method of claim 16 wherein applying heat tosaid second chamber comprises raising said second chamber to a secondtemperature level high than said third chamber temperature level. 21.The method of claim 20 wherein said second temperature is 950° C. 22.The method of claim 16 wherein said temperature level of said thirdchamber is in the range 600 to 1100° C.
 23. The method of claim 16wherein said temperature level of said third chamber is in the range 600to 800° C.
 24. The method of claim 20 wherein said second temperature isin the range of 800 to 1200° C.
 25. The method of claim 16 wherein saidinsulation film comprises an oxide or oxynitride layer.
 26. A method fordistributing a nitrogen containing gas to form an insulation film on asemiconductor, comprising: providing said semiconductor within a mainprocess tube; pre-heating N₂O gas outside said main process tube;delivering said N₂O gas to a gas manifold; splitting said N₂O gas flowthrough said gas manifold to direct said N₂O gas to gas injectorsattached to said main process tube; and, distributing said N₂O gasthrough said injectors to said main process tube housing saidsemiconductor.
 27. The method of claim 26 wherein pre-heating said N₂Ogas comprises raising said N₂O gas to a temperature within the range of850 to 1200° C. outside said main process tube.
 28. The method of claim26 wherein said insulation film comprises an oxide or oxynitride layer.29. An apparatus for forming an insulating layer on a substratecomprising: a furnace including: a first chamber having a first heatingelement for combusting chlorine and steam gas at a first temperature; asecond chamber having a second heating element, said second chamberadapted to receive said chlorine and steam gas and separately a nitrogencontaining gas at a second temperature; and, a third chamber having athird heating element at a third temperature adapted to react separatelysaid gases from said second chamber with said substrate to grow saidinsulating layer.
 30. The apparatus of claim 29 wherein said secondtemperature is higher than said third temperature, such that thecombined gases from said second chamber enter said third chamber at ahigher temperature than said third temperature.
 31. The apparatus ofclaim 29 wherein said first heating element comprises a torch.
 32. Theapparatus of claim 29 wherein said second heating element comprises atorch.
 33. The apparatus of claim 29 wherein said second temperature is950° C.
 34. The apparatus of claim 30 wherein said third temperature isin the range 600 to 1100° C.
 35. The apparatus of claim 30 wherein saidthird temperature level is in the range 600 to 800° C.
 36. An apparatusfor distributing a nitrogen containing gas to form an insulation film ona semiconductor, comprising: a furnace main process tube adapted for gasinjection within said tube and securing said semiconductor; a heatingelement outside said main process tube for pre-heating a nitrogencontaining gas; a gas manifold adapted to receive and deliver saidpre-heated nitrogen containing gas to gas injectors attached to saidmain process tube; said gas injectors adapted to distribute saidnitrogen containing gas to said semiconductor.
 37. The apparatus ofclaim 36 wherein said gas injectors include at least two injectorswithin said main process tube.